In supplying a video signal to for example an image display unit as a liquid crystal display panel unit or the like for image display, and obtaining an image based on the video signal, the correction of level of the video signal by nonlinear processing according to display characteristics of the image display unit is proposed. Such correction of the level (voltage level) of the video signal by nonlinear processing is generally referred to as “gamma correction.”
When the image display unit is formed by a liquid crystal display panel unit for image display, for example, a liquid crystal panel included in the liquid crystal display panel unit displays an image on the basis of a video signal. The image display is in principle effected by change in light transmittance of the liquid crystal panel in response to change in the level of the video signal.
FIG. 2 shows input voltage-light transmittance characteristics indicating a relation between input voltage V and light transmittance T of an example of the liquid crystal panel included in the liquid crystal display panel unit for image display. As is clear at a glance, the input voltage-light transmittance characteristics are nonlinear characteristics. The video signal supplied to the liquid crystal display panel unit that makes image display on the liquid crystal panel having such display characteristics is required to be corrected for level to correct the nonlinear characteristics.
The level correction made on the video signal in conformity to the requirement is gamma correction. Hence, the gamma correction when the liquid crystal display panel unit for image display is used is nonlinear processing correction of the level of the video signal supplied to the liquid crystal display panel unit according to the display characteristics of the liquid crystal display panel unit, or the input voltage-light transmittance characteristics of the liquid crystal panel included in the liquid crystal display panel unit.
FIG. 1 shows an example of a conventional image display apparatus that performs gamma correction of level of a video signal.
In this case, analog/digital (A/D) conversion units 121R, 121G, and 121B digitize a red primary color video signal SR, a green primary color video signal SG, and a blue primary color video signal SB, which form a color video signal, into a digital red primary color signal DR, a digital green primary color signal DG, and a digital blue primary color signal DB, respectively.
The digital red primary color signal DR, the digital green primary color signal DG, and the digital blue primary color signal DB are supplied to a contrast and brightness adjusting unit 122 to be each adjusted in contrast and brightness. Then, a digital red primary color signal DRA, a digital green primary color signal DGA, and a digital blue primary color signal DBA that are adjusted and obtained from the contrast and brightness adjusting unit 122 are supplied to a white balance adjusting unit 123.
In the white balance adjusting unit 123, the digital red primary color signal DRA is adjusted in gain by a gain adjusting unit 124R, and adjusted in direct-current level by a direct-current level adjusting unit 125R, whereby an adjusted digital red primary color signal DRB is obtained from the direct-current level adjusting unit 125R.
The digital green primary color signal DGA is similarly processed by a gain adjusting unit 124G and a direct-current level adjusting unit 125G. Further, the digital blue primary color signal DBA is processed by a gain adjusting unit 124B and a direct-current level adjusting unit 125B.
The digital red primary color signal DRB, the digital green primary color signal DGB, and the digital blue primary color signal DBB thus obtained have relative direct-current levels therebetween set properly. The white balance adjustment is thus made.
The digital red primary color signal DRB, the digital green primary color signal DGB, and the digital blue primary color signal DBB obtained from the white balance adjusting unit 123 are supplied to a gamma correction unit 126.
In the gamma correction unit 126, the digital red primary color signal DRB is subjected to nonlinear level processing by a nonlinear processing unit 127R.
The digital green primary color signal DGB and the digital blue primary color signal DBB are similarly subjected to nonlinear processing by nonlinear processing units 127G and 127B.
The nonlinear processing unit 127R includes a correction signal data table representing nonlinear characteristics in opposite relation to the display characteristics of a liquid crystal display panel unit 118R to be described later, that is, the input voltage-light transmittance characteristics of a liquid crystal panel included in the liquid crystal display panel unit 118R. The nonlinear processing unit 127R sequentially compares the signal level of the digital red primary color signal DRB with the correction signal data table, and then reads corresponding correction signal data. The correction signal data is derived as a digital red primary color signal DRC having a signal level corrected. Thereby the digital red primary color signal DRC derived from the nonlinear processing unit 127R is corrected for signal level by nonlinear processing, that is, gamma-corrected to correct the input voltage-light transmittance characteristics as shown in FIG. 2, for example, of the liquid crystal panel included in the liquid crystal display panel unit 118R.
Similarly, the nonlinear processing unit 127G subjects the digital green primary color signal DGB to gamma correction processing corresponding to a liquid crystal panel included in a liquid crystal display panel unit 118G, and then outputs a digital green primary color signal DGC.
The nonlinear processing unit 127B also subjects the digital green primary color signal DBB to gamma correction processing corresponding to a liquid crystal panel included in a liquid crystal display panel unit 118B, and then outputs a digital green primary color signal DBC.
D/A conversion units 128R, 128G, and 128B convert the digital red primary color signal DRC, the digital green primary color signal DGC, and the digital blue primary color signal DBC that are gamma-corrected and outputted from the gamma correction unit 26 into a gamma-corrected analog red primary color video signal SRC′, a gamma-corrected analog green primary color video signal SGC′, and a gamma-corrected analog blue primary color video signal SBC′. The gamma-corrected analog red primary color video signal SRC′, the gamma-corrected analog green primary color video signal SGC′, and the gamma-corrected analog blue primary color video signal SBC′ are supplied to display driving units 117R, 117G, and 117B, respectively.
Thereby a display driving signal SDR′ based on the red primary color video signal SRC′ is obtained from the display driving unit 117R, and supplied to the liquid crystal display panel unit 118R. Also, a display driving signal SDG′ based on the green primary color video signal SGC′ is obtained from the display driving unit 117G, and supplied to the liquid crystal display panel unit 118G. Further, a display driving signal SDB′ based on the blue primary color video signal SBC′ is obtained from the display driving unit 117B, and supplied to the liquid crystal display panel unit 118B.
The image display apparatus of FIG. 1 further includes: a timing signal generating unit 119 for generating timing signals T1 to T6 on the basis of a horizontal synchronizing signal SH and a vertical synchronizing signal SV; and a PLL unit 120.
The timing signal generating unit 119 supplies the timing signals T1 to T6 to the display driving units 117R, 117G, and 117B and the liquid crystal display panel units 118R, 118G, and 118B, respectively, to operate these parts in predetermined timing.
Thereby the liquid crystal display panel unit 118R is driven by the display driving signal SDR′ from the display driving unit 117R. A red primary color image corresponding to the gamma-corrected red primary color video signal SRC′ is thus displayed on the liquid crystal display panel unit 118R.
Similarly, a green primary color image and a blue primary color image corresponding to the gamma-corrected green primary color video signal SGC′ and the gamma-corrected blue primary color video signal SBC′ are displayed on the liquid crystal display panel units 118G and 118B.
The red primary color image, the green primary color image, and the blue primary color image thus obtained on the liquid crystal display panel units 118R, 118G, and 118B, respectively, are projected in a state of being superimposed on each other on a projection screen via a projection optical system including a projection lens, for example, whereby a color image based on the color video signal formed by the red primary color video signal SR, the green primary color video signal SG, and the blue primary color video signal SB is obtained on the projection screen.
The conventional image display apparatus can make gamma correction, that is, in this case, correct the input voltage-light transmittance characteristics of the liquid crystal panels included in the liquid crystal display panel units 118R, 118G, and 118B. However, the gamma correction in this case is made commonly on pixel data of the digital video signal which pixel data corresponds to each of pixels distributed on the entire image screen obtained on the liquid crystal panel included in each of the liquid crystal display panel units 118R, 118G, and 118B.
That is, the gamma correction based on the same nonlinear characteristics is made on pixel data of the digital video signal which pixel data corresponds to a pixel at a central portion of the image screen obtained on the liquid crystal panel and pixel data of the digital video signal which pixel data corresponds to a pixel at a peripheral portion of the image screen, for example. Such gamma correction cannot correct for differences in the input voltage-light transmittance characteristics according to the position within the screen of the liquid crystal panel.
In addition, the gamma correction does not correct undesired variations in brightness and chromaticity of the red primary color image, the green primary color image, and the blue primary color image thus obtained on the liquid crystal display panel units 118R, 118G, and 118B, respectively, which variations are caused by variations in level of the input video signal, that is, the red primary color video signal SR, the green primary color video signal SG, and the blue primary color video signal SB.
Accordingly, the present applicant has previously proposed a nonlinear processing device and an image display apparatus that can correct the input voltage-light transmittance characteristics in a horizontal and a vertical direction of the screen, that is, according to a position on the screen, and which can further make correction according to signal level (Japanese Patent Application No. Hei 9-271598).
This means further correction of gamma-corrected pixel data according to the position in two-dimensional directions (the horizontal and vertical directions) on the screen and the level. That is, three-dimensional correction is added to gamma correction processing.
The correction in the horizontal and vertical directions is as follows.
FIG. 3 shows a grid block as horizontal and vertical area information for the correction in the horizontal and vertical directions.
The grid block is formed by setting a plurality of areas in a form of a grid in divided units of about 128 pixels, for example, in an X-direction (horizontal direction) and a Y-direction (vertical direction) on the screen. The grid block is formed by correction values C given at points of intersection of horizontal lines and vertical lines.
Suppose that coordinates 0 to p are given in the X-direction, and that coordinates 0 to q are given in the Y-direction, for example. The correction values shown as C(0, 0), C(0, 1), . . . C(p, q) in FIG. 3 are set at coordinates of the points of intersection indicated by dots. Hence, (p+1)×(q+1) correction values are set.
Thereby, (p×q) areas enclosed by coordinates of four intersections (correction values) are formed. The areas are shown as [1, 1], [1, 2], . . . [p, q].
In the correction in the horizontal and vertical directions added to the gamma correction, first an area in such a grid block to which area pixel data belongs is detected. After the area is determined, the position of the pixel data within the area is determined, and then a two-dimensional correction value is calculated from the four correction values forming the area. Thereafter, gamma-corrected pixel data is further corrected by the calculated two-dimensional correction value, whereby the correction according to the horizontal and vertical directions is made possible.
Taking pixel data dxy as an example, it is first determined that the pixel data dxy is included in the area [5, 3], and further where the pixel data dxy is situated within the area [5, 3] is determined.
Since the pixel data dxy is included in the area [5, 3], four correction values C(4, 2), C(5, 2), C(4, 3), and C(5, 3) on the periphery of the area [5, 3] are used to calculate a two-dimensional correction value on the basis of distances of the pixel data dxy within the area [5, 3] from intersection coordinates of the correction values.
Three-dimensional correction is a three-dimensional extension of such two-dimensional correction resulting from addition of signal level on a Z-axis to the two-dimensional correction.
FIG. 4 shows a three-dimensional structure obtained by stacking the grid block of FIG. 3 in the Z-axis direction.
A number of signal level boundaries 0, 1, . . . r are set in the Z-axis direction. A two-dimensional grid block as shown in FIG. 3 is set at each of the level boundaries, whereby a three-dimensional formation of correction values is obtained.
That is, in this case, a correction value C is set at each point of intersection of three-dimensional coordinates, and C(0, 0, 0) . . . C(p, q, r) are set as correction values C. Hence, (p+1)×(q+1)×(r+1) correction values are set.
Further, level blocks L1, L2, . . . Lr are formed between the level boundaries.
A block formed by each of the areas [1, 1] . . . [p, q] as shown in FIG. 3 in the grid block penetrating through the level blocks in the Z-direction is referred to as a position block. FIG. 6 shows a position block A[i, j], which will be described later in detail.
In this case, in the three-dimensional correction in the horizontal and vertical directions and according to level added to the gamma correction, a level block and a position block including the pixel data are first determined.
After the level block and the position block are determined, the level of the pixel data within the level block and the position of the pixel data within the position block are determined, and then a three-dimensional correction value is calculated. In this case, pixel data is situated in a three-dimensional block where a position block and a level block intersect each other. The three-dimensional block is enclosed by eight correction values C. Thus, a three-dimensional correction value corresponding to the pixel data is calculated from the eight correction values according to the position and level of the pixel data within the three-dimensional block. Gamma-corrected pixel data is further corrected by the calculated three-dimensional correction value, whereby the correction in the horizontal and vertical directions and according to the signal level is made possible.
In obtaining a video signal nonlinearly corrected by such techniques previously proposed by the present applicant, the nonlinearly corrected video signal is corrected for undesired variations in brightness and chromaticity according to the horizontal and vertical position on the display screen, and further for undesired variations in brightness and chromaticity of the display screen obtained on the image display unit which variations are caused by variations in level of the original video signal.
However, when the video signal is to be linearly corrected with higher accuracy by applying the techniques previously proposed by the present applicant, there are problems as described in the following which are caused by difference in characteristics between various display devices for the image display unit.
(1) Generally, nonlinear characteristics of output level with respect to input vary with each display device.
There are various kinds of display devices such for example as LCD (Liquid Crystal Display), CRT (Cathode Ray Tube), PDP (Plasma Display Panel), PALC (Plasma Addressed Liquid Crystal), and DLP (Digital Light Processing). They have different nonlinear characteristics.
Also, even individual display devices of the same kind have varying nonlinear characteristics. When a plurality of liquid crystal panels are considered, for example, the plurality of liquid crystal panels have roughly the same nonlinear characteristics, but the nonlinear characteristics vary with each individual liquid crystal panel.
When three-dimensional correction as described above is made in such a situation, the level boundaries in the Z-axis direction are not necessarily set appropriately, so that favorable effects of the three-dimensional correction may not be obtained.
When a nonlinear processing device including the three-dimensional correction is applied to circuit systems of various display devices, for example, the nonlinear processing device cannot deal with difference in nonlinear characteristics of each display device. Even when the nonlinear processing device is included in display devices of the same kind, the nonlinear processing device cannot deal with difference in nonlinear characteristics of each individual display device.
As a result of the above, accuracy of three-dimensional correction for gamma correction can deteriorate.
(2) Image resolution generally varies with each display device.
It is desirable that an upper edge, a lower edge, a left edge, and a right edge of the grid block of correction values in the horizontal and vertical two-dimensional directions coincide with those of an image area.
That is, it is ideal if coordinates (0, 0), (p, 0), (0, q), and (p, q) of four corners of a grid block of FIG. 23, for example, represent four corners of an image area as they are.
When consideration is given to a case where a circuit for making the above-described nonlinear correction is included in image display apparatus as a signal processing system for various display devices, the display devices employed are of course expected to have various screen resolutions. Ideally, it is desirable that size of the grid block be changed according to the resolution to coincide with that of the image area.
However, this requires preparation of enormous amounts of correction values and coordinate values in correspondence with various grid block sizes, thus greatly increasing the scale of the circuit. Thus, the coordinates (and correction values) of the grid block are usually fixed so that display devices of various resolutions are dealt with by a single grid block.
This, however, results in a vertically and horizontally asymmetric relation between the grid block and the image area. Thus, correction of nonlinear characteristics in the two-dimensional directions may result in an unnatural image state.
When a nonlinear correction circuit in which a grid block provided for a device with a high resolution is set is incorporated in a signal processing system for a display device with a low resolution, for example, a relation between the grid block and the image area is as shown in FIG. 19A.
That is, since the grid block and the image area are made to correspond to each other by using coordinates (0, 0) as an origin, amounts of displacement between the grid block and the image area are asymmetric in both the horizontal direction and the vertical direction. This results in an unnatural image.